The blood system is maintained by a small pool of haematopoietic stem cells (HSCs), which are required and sufficient for replenishing all human blood cell lineages at millions of cells per second throughout life. Megakaryocytes in the bone marrow are responsible for the continuous production of platelets in the blood, crucial for preventing bleeding--a common and life-threatening side effect of many cancer therapies--and major efforts are focused at identifying the most suitable cellular and molecular targets to enhance platelet production after bone marrow transplantation or chemotherapy. Although it has become clear that distinct HSC subsets exist that are stably biased towards the generation of lymphoid or myeloid blood cells, we are yet to learn whether other types of lineage-biased HSC exist or understand their inter-relationships and how differently lineage-biased HSCs are generated and maintained. The functional relevance of notable phenotypic and molecular similarities between megakaryocytes and bone marrow cells with an HSC cell-surface phenotype remains unclear. Here we identify and prospectively isolate a molecularly and functionally distinct mouse HSC subset primed for platelet-specific gene expression, with enhanced propensity for short- and long-term reconstitution of platelets. Maintenance of platelet-biased HSCs crucially depends on thrombopoietin, the primary extrinsic regulator of platelet development. Platelet-primed HSCs also frequently have a long-term myeloid lineage bias, can self-renew and give rise to lymphoid-biased HSCs. These findings show that HSC subtypes can be organized into a cellular hierarchy, with platelet-primed HSCs at the apex. They also demonstrate that molecular and functional priming for platelet development initiates already in a distinct HSC population. The identification of a platelet-primed HSC population should enable the rational design of therapies enhancing platelet output.

The stepwise commitment from hematopoietic stem cells in the bone marrow (BM) to T lymphocyte-restricted progenitors in the thymus represents a paradigm for understanding the requirement for distinct extrinsic cues during different stages of lineage restriction from multipotent to lineage restricted progenitors. However, the commitment stage at which progenitors migrate from the BM to the thymus remains unclear. Here we provide functional and molecular evidence at the single cell level that the earliest progenitors in the neonatal thymus possessed combined granulocyte-monocyte, T and B lymphocyte, but not megakaryocyte-erythroid lineage potential. These potentials were identical to those of thymus-seeding progenitors in the BM, which were closely related at the molecular level. These findings establish the distinct lineage-restriction stage at which the T lineage commitment transits from the BM to the remote thymus.

The gene encoding the transcriptional co-activator MN1 is the target of the reciprocal chromosome translocation (12;22) (p13;q12) in some patients with acute myeloid leukemia (AML). In addition, expression array analysis showed that MN1 was overexpressed in AML specified by inv(16), in some AML overexpressing ecotropic viral integration 1 site (EVI1) and in some AML without karyotypic abnormalities. Here we describe that mice receiving transplants of bone marrow (BM) overexpressing MN1 rapidly developed myeloproliferative disease (MPD). This BM also generated myeloid cell lines in culture. By mimicking the situation in human inv(16) AML, forced coexpression of MN1 and Cbfb-SMMHC rapidly caused AML in mice. These findings identify MN1 as a highly effective hematopoietic oncogene and suggest that MN1 overexpression is an important cooperative event in human inv(16) AML.

In the present paper, we have examined whether human tissue inhibitor of metalloprotease-1 (hTIMP-1) is able to exert a growth factor-like effect on two clonal cell lines (BC-3A and BC-61), isolated from a parental line of human breast carcinoma cells (8701-BC), and endowed with different growth and invasive behaviour 'in vitro' and in nude mouse. The data obtained indicate that only the more tumorigenic clonal cell line (BC-61) is responsive to hTIMP-1 treatment by increasing its proliferative rate in a dose-dependent manner. It was also found that BC-61 cells selectively express a transmembrane protein of about 80 kDa able to bind hTIMP-1 'in vitro' and 'in vivo' with high affinity (Kd of 0.07 +/- 0.004 nM), and that treatment of BC-61 cells with a proliferation-promoting concentration of hTIMP-1 is able to stimulate tyrosine-targeted phosphorylation. The cumulative results obtained strongly support the hypothesis that hTIMP-1, 'classically' regarded as a collagenase inhibitor, may be a crucial element of the extracellular signalling network during breast cancer development by controlling cell growth phenotype in autocrine and paracrine manner, and that intratumoural heterogeneity for the biological response to TIMP-1 may exist within the composite cell population of the primary tumour site.

TEL2/ETV7 is highly homologous to the ETS transcription factor TEL/ETV6, a frequent target of chromosome translocation in human leukemia. Although both proteins are transcriptional inhibitors binding similar DNA recognition sequences, they have opposite biologic effects: TEL inhibits proliferation while TEL2 promotes it. In addition, forced expression of TEL2 but not TEL blocks vitamin D3-induced differentiation of U937 and HL60 myeloid cells. TEL2 is expressed in the hematopoietic system, and its expression is up-regulated in bone marrow samples of some patients with leukemia, suggesting a role in oncogenesis. Recently we also showed that TEL2 cooperates with Myc in B lymphomagenesis in mice. Here we show that forced expression of TEL2 alone in mouse bone marrow causes a myeloproliferative disease with a long latency period but with high penetrance. This suggested that secondary mutations are necessary for disease development. Treating mice receiving transplants with TEL2-expressing bone marrow with the chemical carcinogen Nethyl-N-nitrosourea (ENU) resulted in significantly accelerated disease onset. Although the mice developed a GFP-positive myeloid disease with 30% of the mice showing elevated white blood counts, they all died of T-cell lymphoma, which was GFP negative. Together our data identify TEL2 as a bona fide oncogene, but leukemic transformation is dependent on secondary mutations. IntroductionThe ETS (E26 transformation specific) proteins belong to a large family of eukaryotic transcription factors (TFs) unique to the metazoan lineage and highly conserved throughout evolution. 1,2 Some ETS proteins are expressed ubiquitously while others show a restricted tissue distribution. 3,4 All ETS proteins possess a highly conserved 85-amino acid (aa) ETS domain that binds a purine-rich GGAA/T core motif present in promoters and enhancers of different target genes. 4,5 A subgroup of ETS TFs, including the TEL proteins, also contains a conserved pointed (PNT) protein-protein interaction domain, which mediates the formation of homodimers/ oligomers 6,7 and heterodimers/oligomers. 8,9 In TEL this domain is involved in transcriptional repression. 10 Vertebrate ETS TFs are implicated in many aspects of normal development and differentiation, including that of the hematopoietic system. 11 For example, Ets-1, Fli-1, and Erg are expressed early during mouse development in the blood islands of the yolk sac where hemangioblasts are present. 12 A high level of Pu-1 expression in CD34 ϩ hematopoietic progenitors directs their differentiation toward the macrophage lineage, while low Pu-1 expression induces B-cell differentiation. 13 The expression of IL-7R␣, a receptor essential for pro-B-cell development, is directly regulated by Pu-1 in lymphoid progenitors. 3,14 The ubiquitously expressed TEL/ETV6 is essential for normal embryonic yolk sac angiogenesis and is important for the maintenance of the adult bone marrow microenvironment. 15 Several ETS genes, including ETS1, ETS2, PU1, FLI1, TEL/ ETV6, and ERG, possess oncog...